1. Field of the Invention
The present invention relates generally to boiling water reactors and, more particularly, to a method and apparatus for repairing welds in a boiling water reactor shroud.
2. Discussion of the Related Art
Boiling water reactor shrouds are disposed concentrically within reactor vessels to divide the flow of coolant water through the reactor vessels and to structurally support and align the fuel assemblies, steam separator assemblies and control rod guide tubes. The shrouds are generally cylindrical and are typically formed of multiple arcuate steel plates joined by welds along their abutting vertical and horizontal edges. After periods of use, cracking of the shroud within heat affected zones of the welds tends to occur as a result of corrosion, radiation and stress. When excessive cracking of the welds occurs, the shroud must either be replaced or repaired. U.S. Pat. No. 5,402,570 to Weems et al, the disclosure of which is incorporated herein by reference, describes a method of repairing boiling water reactor core shrouds having horizontal cracks in heat affected zones of welds by securing plural tie rods in vertical orientation about the periphery of the cracked shroud to axially compress the shroud and thereby urge the opposing surfaces of the horizontal cracks toward one another. Lateral or radial spacers or bumpers may also be installed between the inner surface of the reactor vessel and the outer surface of the shroud and/or tie rods to resist horizontal seismic loads and to hold the shroud in place relative to the vessel wall in the event of cracking along any horizontal weld during a seismic event.
While the tie rod repair described in U.S. Pat. No. 5,402,570 provides an acceptable method for mitigating the consequences of cracking along horizontal welds, there exists a need for a vertical weld repair which can be used alone or in combination with tie rods to mitigate the consequences of cracking along vertical welds in a boiling water reactor core shroud.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for repairing cracked vertical welds in a boiling water reactor core shroud.
The repair according to the present invention involves forming a pair of slots in a core shroud on opposite sides of a crack in angled relation to one another so that the lateral spacing between the slots increases in a direction parallel to the crack, lowering a clamp with laterally spaced slides into the annular space between the core shroud and the reactor vessel, positioning the slides in the slots, and moving the clamp in the direction of increasing lateral spacing between the slots to prevent unacceptable opening of the crack during reactor operation by providing a structural load path which transmits structural loads across the crack. Preferably, the slides are toed laterally inward or toward one another and the slots extend into the shroud at oblique angles relative to the shroud surface to mate cooperatively with the inwardly toed slides to provide a locking feature which prevents the clamp from falling out of the slots. When repairing vertical welds in this manner, the slots are oriented so that the lateral spacing between the slots increases in the downward direction. The clamp can then be moved downwardly until it is tightly seated and will tend to remain tight due to gravity and flow loads which continue to exert downward forces on the clamp. The clamp can also be provided with locking members such as screws which extend from the clamp in a direction opposite the direction of increasing lateral slot spacing to hold the clamp in place by contacting walls of the slots.
Some of the advantages of the vertical weld repair according to the present invention are that the repair can be performed without unloading the reactor fuel, that the repair can be performed in combination with other types of repairs such as those involving installation of tie rods, that the repair can be designed to meet all operating and design basis loads for a reactor, that the repair eliminates the need to inspect the repaired weld thereby minimizing in-service inspection of the shroud, that the repair can mitigate the effects of cracking along welds without preloading, that the repair can accommodate cold feedwater injection, that the repair provides installation clearance take-up while holding opposed surfaces of a crack together for normal operation, that the repair can accommodate cracking well away from the weld as needed, that stresses on the shroud are minimized, that interference with reactor internals such as the top guide and core plate is minimized, that welds behind core spray piping and other structures in the reactor vessel can be repaired, that the repair will not loosen in service or create any shroud leak paths, and that the repair can be performed using no loose pieces in a cost effective and timely manner.
Other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference numerals.